1. Field of the Invention
This invention relates to porous polyolefin membranes suitable for use in precision filtration and having excellent heat resistance; porous polyolefin membranes suitable for use in precision filtration and having both excellent heat resistance and hydrophilicity; and processes for the production of such membranes.
2. Description of the Prior Art
In recent years, a variety of separating membranes have come to be used in various fields such as water purification, blood treatment, air cleaning and food industry. For example, precision filtering membranes formed of a polyolefin such as polyethylene are being widely used to easily obtain highly pure water and highly clean air, because they have excellent chemical resistance and excellent physical properties such as strength, elongation and flexibility. As the range of use of precision filtering membranes extends, it is more strongly desired to use them at elevated temperatures, for example, of the order of 80.degree. to 95.degree. C.
Moreover, in many applications (such as food industry and blood treatment) of precision filtering membranes, their contamination with microorganisms such as bacteria and molds is not permitted and, therefore, they are sterilized prior to use. Useful sterilizing processes include treatment with chemicals such as ethylene oxide, formalin and hydrogen peroxide, exposure to radiation such as gamma rays, and heat treatment such as steam heating. Among them, steam heating is most preferred so long as the precision filtering membrane being treated can withstand it, because it is an effective and simple process involving little risk of contamination with impurities during the sterilization step. Usually, steam heating comprises treatment with steam at 121.degree. C. for 30 minutes.
However, porous membranes formed of polyolefins such as polyethylene tend to undergo marked heat shrinkage and morphological changes, when they are continuously used at an elevated temperature of 80.degree. to 95.degree. C. or sterilized by steam heating comprising treatment with steam at 121.degree. C. for 30 minutes. This often causes a sharp reduction in water or air permeability and impairs their function as precision filtering membranes.
Moreover, since porous polyolefin membranes are hydrophobic, water cannot permeate therethrough when they are used as such. Accordingly, where it is desired to utilize them for the treatment of aqueous liquids (i.e., water and water-based solutions), they need to be hydrophilized in advance by means of a hydrophilizing agent such as an alcohol or the like. However, porous polyolefin membranes hydrophilized with an alcohol or the like have the disadvantage that, once they are dried, their hydrophilicity is lost.
In an attempt to overcome the above-described disadvantages of porous membranes formed of polyolefins, Japanese Patent Laid-Open No. 33878/'87 has proposed a technique for improving the heat resistance of porous polyolefin membranes. This technique comprises forming a heat-resisting polymer film having a crosslinked structure on the surfaces of polyolefin hollow fiber membranes.
Moreover, a number of techniques for imparting hydrophilicity to porous polyolefin membranes have also been proposed. Specifically, Japanese Patent Laid-Open No. 57836/'81 discloses porous polyethylene membranes which have been rendered hydrophilic by the introduction of sulfonic acid groups; Japanese Patent Laid-Open No. 271003/'86 discloses porous polyolefin membranes which have been rendered hydrophilic by causing membranes to hold an ethylene-vinyl alcohol copolymer on their surfaces, Japanese Patent Laid-Open No. 125408/'86 discloses porous polyethylene hollow fiber membranes which have been rendered hydrophilic by causing the membranes to hold an ethylene-vinyl acetate copolymer on their surfaces and then saponifying it.
Furthermore, as a technique for imparting both heat resistance and hydrophilicity simultaneously to porous polyolefin membranes, Japanese Patent Laid-Open No. 59030/'90 has proposed a method which comprises causing the membranes to hold a crosslinked polymer composed of a styrene monomer and divinylbenzene on their surfaces and then polymerizing a hydrophilic monomer (such as diacetone acrylamide) and a crosslinkable monomer thereon to form a hydrophilic copolymer. In addition, Japanese Patent Laid-Open No. 69673/'91 discloses a method which comprises thermally polymerizing a mono- or di(meth)acrylate of polyethylene glycol and a multifunctional (meth)acrylate on the pore surfaces of a porous polyolefin membrane to bond the resulting copolymer firmly thereto.
However, in the method proposed by Japanese Patent Laid-Open No. 33878/'87, it has been found that the heat resistance of porous membranes is improved temporarily, but some physical properties (such as elongation at break and breaking strength) inherently possessed by the porous membranes are deteriorated while they are immersed in hot water for a long period of time.
In the method for imparting both heat resistance and hydrophilicity as described in Japanese Patent Laid-Open No. 59030/'90, the heat resistance of porous polyolefin membranes is improved owing to the use of a crosslinked polymer derived from aromatic monomers, but the resulting membranes have some disadvantages. Specifically, the crosslinked polymer tends to be so rigid that the flexibility inherently possessed by hollow fiber membranes may be impaired. Moreover, when the membranes are immersed in hot water for a long period of time, their mechanical properties such as elongation at break are reduced, probably because of the relatively poor affinity between the crosslinked polymer and the material forming the membranes per se.
In the method described in Japanese Patent Laid-Open No. 69673/'91, the resulting membranes have been found to have the disadvantage that, probably because the monomers to be copolymerized have high molecular weights and are hence bulky hydrophilic monomers, the adhesion of the copolymer to the polyolefin hollow fiber membrane matrix is not satisfactorily good, and the heat resistance of the membranes cannot be maintained for a long period of time when they are immersed in hot water. Thus, a technique for imparting both heat resistance and hydrophilicity which can withstand long-time use also in hot water having a temperature of about 90.degree. C. has not been known in the prior art.